The present invention relates to thermoformed components such as energy absorbers for bumpers, and more particularly to a thermoformed energy absorber with integrally formed attachment structure and other coined structures for attachment to a reinforcement beam in a vehicle bumper system. However, the present invention is not contemplated to be limited to only energy absorbers, nor to only vehicle bumper systems. The present invention further relates to thermoform processes and apparatus for forming integral attachment structures and/or other coined structures of different thickness than the initial thickness of a thermoformed sheet.
Thermoforming processes offer the ability to reform flat thermoplastic sheet material into a three-dimensional sheet shape at low cost and with relatively short lead times and low cost for tooling. However, thermoformed products are limited to a thickness of the initial sheet, with formed areas often being stretched to thicknesses considerably less than the initial sheet thickness. Notably, thermoforming processes become more difficult as sheet thicknesses increase, because the sheets become increasingly difficult to uniformly heat, to control while hot, to uniformly shape, and to control while cooling. In particular, the sheets become difficult to accurately control to a final shape due to complex stress patterns that result from stretching, forming, and cooling (including stress caused by stretching and by non-uniform cooling due to location and/or three-dimensional shapes). Further, tooling and apparatus for thermoformed components are generally limited to vertical movement, such that undercuts, blind surfaces, and parts with non-uniform thickness and complex shapes are not possible. As a result, “complex” parts are usually injection molded.
Relatively “weak” releasable thermoformed snap features have been formed in thin gauge thermoplastic material by thermoforming, such as deli trays whose nominal base thickness is less than 1 mm, where the snap feature includes a thin-walled hollow protrusion (i.e., considerably less than 1 mm wall thickness of the initial sheet due to stretching during the thermoforming process). Increasing the sheet thickness is typically not possible in these applications, both due to cost and also because these snap features become dramatically more difficult to control as the raw material sheet thickness increases beyond 1 mm. However, “hollow post” snap features formed in a 1 mm sheet have a high variability in the amount of retention force. This can render the feature non-functional for long term retention due to a highly-variable inconsistent retention force. There are instances where such features are used to temporarily adhere one-half of a “clamshell” to another. However, snapping into small holes or slots in a mating component has proven to be an even bigger challenge for thermoformed products since only the material directly above the feature is available for forming. Specifically, once the material is pulled into these small features by the vacuum and pressures of traditional/known thermoforming processes, the walls are typically so thin that little engagement or retention force is supplied by the snap. Further, traditional thermoforming processes are unable to form substantial undercuts and structures with blind surfaces. Alignment of mating connecting features is also problematic since it is difficult to accurately control hole locations in a thermoformed sheet. For all of these reasons, permanent attachment features have often been adhered to thermoformed product through expensive secondary operations, such as by various plastic joining methods such as glue, sonic welding, melt bonding, hot plate welding, heat staking, vibration welding, RF welding, and the like. These secondary operations often make the thermoformed product too expensive as compared to other manufacturing methods, such as injection molded thermoplastic products for high volume applications.
Thermoforming processes include additional limitations. For example, it is difficult and/or impossible to efficiently, reliably, easily and accurately locate trim holes formed in secondary hole-forming operations in the thermoformed part in relation to the formed “snap-in” features on the part. Trimming holes in-line requires a robust system to remove slugs of material on every single part, which has historically proven very difficult. Secondary operations, even those done outside of the thermoforming line, are more reliable, but result in an added tooling and processing expense that often makes the thermoformed product too expensive and uncompetitive as compared to other manufacturing methods, such as injection molded thermoplastic materials, particularly for high volume applications. These secondary operations include laser trimming, water jet trimming, NC cutting, matched metal trim dies, contoured kiss cut dies and the like.
As noted above, formed thermoformed snap features for thin gauge materials, less than 1.0 mm in base starting thickness, have been used to attach deli “clamshell” tray features. The most commonly used feature is a male formed cone on one-half of the clamshell with minimal draft angle (1-5 degrees) and a female formed square depression with minimal draft angle (1-5 degrees) that has rounded corners. A “shallow” undercut on the non-tool side of the plastic sheet of both the male and female features forms naturally as a result of the thermoforming process. When the two halves are snapped together, the formed features typically provide a retention force that is just about equal to but slightly higher than the engagement force. The snap is engaged at the tangent points where the round male meets the square sides of the female depression. Due to the light gauge of the base material, the straight walls of the female depression flex outward during insertion and extraction. This snap is effective for applications where one may want to snap and unsnap the product several times while filling the tray and removing item from it. However, it is generally unsuitable where a permanent attachment or an easy-to-assembly difficult-to-remove attachment is required.
Muirhead U.S. Pat. No. 6,718,888 describes an alternative methodology to relying on natural undercuts in the part to adhere two separate plastic halves. Muirhead '888 concerns an effort to produce a thermoformed pallet assembly where the two halves are snapped together by integral structures. Muirhead '888 describes incorporating “action” into one or both halves of the tool to form an undercut while the tool is closed. The undercut retracts prior to ejection in order to avoid a “die lock” condition where the snap feature could be elongated or distorted during the ejection process. A problem is that the mechanism is relatively complex, expensive to construct, difficult to control during the forming process, and difficult to maintain. Further, the mechanism relies on air pressure and vacuum to pull the material into the undercut feature, which limits the geometries one can select from for making snap attachments. Further, the snap feature that is formed has a thickness limited to a thickness of the starting sheet material. Further, it is difficult to control the wall stock of the snap feature since the forming process is essentially one sided, including a difficulty in controlling a final shape and position of the snap feature.